1. Field
This invention pertains to a welding arrangement and process for joining parts in various weld configurations. More particularly, the process relates to the use of a resistively heated cover sheet that provides conductive heating of the weld zone of the parts to be welded and a lateral compressive force device that constrains the molten weld pool to the weld zone and ensures complete filling of the weld zone with non-porous weld material.
2. Background
As consumer demand for more fuel efficient vehicles increases, as well as increased government regulation of fleet fuel economy, the use of light-weight materials such as aluminum for automotive components is becoming more attractive. As part of this effort, aluminum parts with continuous joints are of interest; however, the processes used to fabricate these joints are expensive. These processes include laser welding, gas tungsten arc (GTA) welding, and electron beam welding.
Meadowcroft (U.S. Pat. No. 1,560,080) uses a method of conductive spot welding in which aluminum arranged in a lap joint configuration is spot welded by placing two steel sheets on opposite sides of the lap joint configuration. Electrodes placed in contact with the steel sheets are used to resistance heat the steel sheets which in turn conductively heat a spot in the aluminum lap joint configuration and join the aluminum by means of one or more welded spots along the lap joint configuration. Such a technique is effective in that the molten aluminum in the spot configuration is confined on all sides, i.e., on the top and bottom by the steel sheets and on all sides by the relatively cold aluminum sheet. It is specifically noted that the welding does not occur at the edges of the aluminum sheet but inward from all edges so as to afford a lateral constraint about the aluminum weld pool. When the Meadowcroft technique is applied to continuous seam welding, i.e., a weld configuration in which the edges of the sheets are butted against each other to form a bond line and the edges welded together, the weld tends to be less than satisfactory. In a seam welding configuration, the bond line becomes a discontinuity in the constraining material which causes a progressive separation of the sheets at the bond line as the welding progresses and subsequent loss of joint quality. Such loss in joint quality includes (progressively) reduction in weld (joint) thickness, loss of joint material, severe internal weld porosity, and finally total failure of the ability to form a bond.
Thus it is an object of the present invention to provide a high quality, continuous seam weld by means of conductive heating.
It is an object of the present invention to provide a lateral compressive force to the weld zone of a seam weld.
It is an object of the present invention to maintain joint thickness.
It is an object of the present invention to provide an inexpensive continuous weld.
It is an object of the present invention to provide conductive heating of a weld zone.
It is an object of the present invention to prevent metal expulsion from the weld.
It is an object of the present invention to provide an improved method of welding aluminum.
It is an object of the present invention to provide a weld of good weld integrity.
It is an object of the present invention to reduce electrode wear rates.
It is a further object of the present invention to provide a weld with minimal evidence of cracking or porosity.
To meet these objects, the present invention features placing two electrically-conducting parts to be joined, typically metals although electrically conductive plastics may also be used, juxtaposed one to the next to form a weld zone. One or more covering sheets of a higher-melting, electrically-conductive material than the parts to be joined is placed next to the weld zone. Oppositely charged electrodes are positioned and aligned to cause a current to flow through the covering sheet and the weld zone of the materials to be joined. Resistance heating of the cover sheet(s) with subsequent conductive heating of the weld zone produces sufficient heat to melt and weld the parts together. The covering sheet(s) not only provides conduction heating of the weld zone but it also provides constraint to the molten metal to prevent expulsion of molten part materials from the weld pool. For butt weld type configurations, the use of a force applying device to apply a compressive lateral force to the weld zone during conductive heating substantially increases the weld efficiency of the joint.
In a common arrangement used with the present invention, the weld zone of the parts to be joined is sandwiched between two covering sheets with oppositely charged electrodes then placed in contact with the covering sheets. By using rotating circular electrodes, the parts can be joined in a continuous seam weld. However, it is to be realized that the method may also be used for intermittent and spot welds using appropriate electrode configurations.
A wide variety of materials can be joined including aluminum, lead, copper, brass and other alloys provided the materials to be joined melt are at a lower temperature than the covering material. The cover sheet typically is a steel such as SAE 1010/1008 but can include various other covering materials including cobalt, nickel, and titanium based alloys that melt at a temperature greater than the parts to be joined. Various weld-zone configurations may also be used including butt, tee, lap and mash configurations.
For butt joint and similar edge-joining, continuous-weld configurations, it has been found especially beneficial to use a force application device to apply a lateral force to the weld so as to urge the edges of the sheets to be joined together at the bond line(weld zone) during the welding process. Such a feature has the advantages of: 1) preventing separation of the sheets at the bond line, i.e., failure of the ability to form a bond, 2) reducing the internal porosity of the resulting joint, 3) preventing loss of joint material, 4) preventing a reduction in joint thickness, and 5) substantially improving the joint efficiency of the resulting weld.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in procedures, structural features and arrangement of parts may appear to a person skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.